Cellular communication system and re-use pattern therefor

ABSTRACT

A cellular communication system has a frequency bandwidth arranged into a plurality of frequency channels, and a plurality of neighbouring first ( 130 ) and second ( 132 ) sites each having sectors (a 1 -f 1 , a 2 -f 2 ) containing at least one frequency channel. Corresponding sectors in each of the neighbouring first ( 130 ) and second sites ( 132 ) have consecutive frequency channels from the frequency bandwidth, thereby producing a two-site re-use pattern ( 134, 136 ). The cellular communication system may be adapted to support an underlay/overlay cell configuration in which neighbouring first ( 230 ) and second ( 232 ) sites each have six-sectors containing at least one frequency channel (b 1 -b 12 ) of a two-site repeat pattern. The six sectors further each contain at least one frequency channel (t 1 -t 6 ) of a one-site repeat pattern. Corresponding sectors in each of the neighbouring first ( 130 ) and second sites ( 132 ) have consecutive frequency channels in the two-site repeat pattern and identical channels in the one-side repeat pattern.

BACKGROUND OF THE INVENTION

This invention relates, in general, to a cellular communication systemand is particularly, but not exclusively, applicable to a repeat (orre-use) pattern for such cellular communication system. Moreparticularly, an aspect of the present invention may be employed in acellular communication system having concentric cells.

SUMMARY OF THE PRIOR ART

As a consequence of the limited availability of frequency bandwidth forcellular communication systems generally, such as the pan-EuropeanGlobal System for Mobile (GSM) cellular communication, designers mustemploy frequency re-use techniques to optimise and increase cellularsystem capacity. More explicitly, a frequency bandwidth that is assignedto the communication system is divided into many channels that arethemselves attributed to frequency groups. These frequency groups arethen individually allocated to sectors that form a site (or cell), withthe deployment of one set of frequency groups across many sites defininga cluster of sites within the communication system. As such, cellplanning represents the distribution of frequency groups between anumber of sectors in a cluster, while a repeat (or re-use) pattern forthe system is indicated by the relationship between the number of sitesthat are covered by an integer number of sets of frequency groups. Forexample, a repeat pattern of two (2) would be achieved from thedeployment of two complete sets of frequency groups to cover a clustercontaining 4 sites (with each site typically containing either three (3)or six (6) sectors).

To date, repeat patterns offering a two-site repeat have proveddifficult to implement because of co-channel interference and, inparticular, considerable adjacent channel interference (or splatter)prevalent in current repeat patterns. In these respects and as will beunderstood, co-channel interference occurs when different sectors usethe same frequency groups (with an intensity for the co-channelinterference determined by the proximity and number of co-channelcells), whereas adjacent channel interference occurs as a result of theadjacent location of contiguous frequency bands (channels). Clearly, ina two-site repeat pattern, adjacent channel interference becomes anincreasing problem.

At present, therefore, many systems (including GSM and DigitalCommunication System (DCS) 1800) utilise a single-layer four-site repeatpattern.

In an attempt to mitigate against the effects of adjacent channelinterference in a two-site repeat pattern, “frequency hopping” schemes(such as envisaged in Code Division Multiple Access (CDMA)) offer apotential solution to the problem, although such schemes are relativelycomplex and commercially expensive (because of the necessity forsophisticated hand-over algorithms and the increased complexity ofsystem infrastructure). As such, allocation of frequency groups tosectors may be on either a permanent or initial basis.

Furthermore, in an endeavour to optimise cellular systems still further,designers have recently begun to experiment with concentric cellarrangements in which a first set of frequencies having a first repeatpattern is over-layed by a second set of frequencies having a secondrepeat pattern. In this respect, systems have emerged that provide afour-site (by three-sector) repeat pattern with a one-site (by threesector) underlay/overlay, notwithstanding that the use of a one-siterepeat pattern for a site having three sectors inherently producesadjacent channel interference (or splatter) in the one-site repeatpattern (that is potentially unacceptable to network operators).

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided acellular communication system having a frequency bandwidth arranged intoa plurality of frequency channels, the cellular communication systemcomprising neighbouring first and second sites each having sectorscontaining at least one frequency channel, wherein corresponding sectorsin each of the neighbouring first and second site have consecutivefrequency channels from the frequency bandwidth, thereby producing atwo-site re-use pattern.

In a preferred embodiment, neighbouring sectors within both the firstand second sites observe a next but one frequency channel relationshipwith adjacent frequency channels in at least one neighbouring sector.

Indeed, by providing clusters having corner illuminated, six-sectoredsites, the preferred embodiment of the present invention provides atwo-site repeat pattern that avoids adjacent channels and hence adjacentchannel splatter.

In another preferred embodiment that supports an underlay/overlay cellconfiguration, the frequency bandwidth is arranged into a plurality offrequency channels for use in a two-site repeat pattern and a one-siterepeat pattern of the cellular communication system, the cellularcommunication system further comprising neighbouring first andsix-sectored sites in which each sector contains at least one frequencychannel from the two-site repeat pattern and at least one frequencychannel from the one-site repeat pattern, wherein corresponding sectorsin each of the neighbouring first and second six-sectored sites haveconsecutive frequency channels of the two-site repeat pattern andidentical frequency channels of the one-site repeat pattern.

Neighbouring sectors within both the first and second six-sectored sitesobserve a next but one frequency channel relationship with adjacentfrequency channels in at least one neighbouring sector for the two-siterepeat pattern.

Furthermore, in this overlay/underlay embodiment, it is preferable thata first half of the neighbouring sectors within both the first andsecond six-sectored sites observe a next but one frequency channelrelationship with adjacent frequency channels in at least oneneighbouring sector for the one-site repeat pattern and a second half ofthe neighbouring sectors within both the fist and second six-sectoredsites observe a next but three frequency channel relationship withadjacent frequency channels in at least one neighbouring sector for theone-site repeat pattern.

The system of this embodiment of the present invention controlshand-over of a call from the one-site repeat pattern to the two-siterepeat pattern in response to a relatively high interference level forthe call on the one-site repeat pattern.

In a second aspect of the present invention there is provided a cellularcommunication system having a frequency bandwidth arranged into aplurality of frequency channels that are distributed on a consecutivefrequency channel basis amongst a plurality of frequency groups, thecellular communication system comprising a cluster having neighbouringfirst and second six-sectored sites, each sector of each site containinga frequency group having at least one frequency channel and wherein: i)the first six-sectored site comprises a first frequency channel and atleast five other frequency channels each having an integer multiple nextbut one frequency channel relationship to the first frequency channel,wherein neighbouring sectors in the first six-sectored site each containa frequency group having respective frequency channels that observe anext but one frequency channel relationship with frequency channels inat least one neighbouring frequency group; ii) the second six-sectoredsite comprises a second frequency channel, consecutive in frequency tothe first frequency channel, and at least five other frequency channelseach having an integer multiple next but one frequency channelrelationship to the second frequency channel, wherein neighbouringsectors in the second six-sectored site each contain a frequency grouphaving respective frequency channels that observe a next but onefrequency channel relationship with at least one neighbouring frequencygroup; and iii) consecutive frequency channels of the frequencybandwidth are assigned to corresponding sectors in each of theneighbouring first and second six-sectored sites to produce a two-sitere-use pattern for the cluster in which consecutive frequency channelsare alternated between the first and second six-sectored sites.

Exemplary embodiments of the present invention will now be describedwith reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art cellular communication system.

FIG. 2 shows a sectorised site typically utilised in the prior artcellular communication system of FIG. 1.

FIG. 3 shows a typical allocation of frequency bandwidth for the priorart cellular communication system of FIG. 1.

FIG. 4 illustrates a 2-site repeat pattern according to a preferredembodiment of the present invention.

FIG. 5 illustrates a 2-site repeat pattern according to an alternateembodiment of the present invention.

FIG. 6 shows a typical allocation of frequency bandwidth for a prior artcellular communication system that utilises a concentric cell pattern.

FIG. 7 illustrates a two-site repeat pattern with a one-siteunderlay/overlay repeat scheme according to another embodiment of thepresent invention.

FIG. 8 illustrates a two-site repeat pattern with a one-siteunderlay/overlay repeat scheme according to a further embodiment of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a typical prior art cellular communications system 10 inwhich a coverage area is defined by a number of sites (or cells) 12-22represented in conventional hexagonal fashion. Each site 12-22 has abase station 24-34 responsible for controlling communication traffic inthe respective site. Typically, the base stations 24-34 will becentrally located, although other positions may be desirable subject tosurrounding terrain or propagation conditions. As will be understood,each base station 24-34 may receive 38 and/or transmit 40 signalsfrom/to mobile communication devices 42-46 that roam throughout thecommunication system 10. Furthermore, each base station (BS) 24-34 isresponsive to an operations and maintenance centre (OMC) 49 arranged tohave overall system control, which OMC 49 may be either on a regional orsystem basis (dependent upon the size of the communications system 10).

Each site 12-22 of the cellular communication system 10 of FIG. 1, istypically partitioned into six operational sectors 50-60 (as shown inFIG. 2), with each sector 50-60 serviced by one or more radio channelunits (RCUs) 62-72 and associated transmit and/or receive antennas74-84, respectively. Typically, although not expressly shown, it will beappreciated that some redundancy in the number of RCUs is provided tothe site to prevent loss of a sector if failure of a particular RCUoccurs. Furthermore, it is usual for Broadcast Control Channels (BCCHs)and Traffic Channels (TCHs) to be served by specific and individual RCUswithin each sector. As interference performance in a one-site repeatpattern is relatively high under a fully-loaded system, system load istypically reduced to diminish system interference. As such, cellularsystems employing concentric cell arrangements will typically comprisemore RCUs that conventional cellular systems.

As previously explained, a cellular communication system has a limitedfrequency bandwidth 90 (shown in FIG. 3) that is divided into manycontiguous frequency channels 91-97 (having equal portions of theavailable frequency bandwidth). In this respect, coincidence may dictatethat the number of frequency channels for the system corresponds to thenumber of frequency groups, whereby each frequency group contains asolitary channel, however this is seldom the case. Therefore, channelsare usually allocated (from a consecutive series of channels) tofrequency groups on an incrementing and rotational basis, whereby afirst channel 91 is assigned to a first frequency group, a secondchannel 92 is assigned to a second frequency group, . . . an n^(th)channel 95 is assigned to a final frequency group, an (n^(th)+1) channel96 is again assigned to the first frequency group, an (n^(th)+2) channel97 is again assigned to the second frequency group, and so on (in acyclic fashion) until all available channels for the frequency bandwidthhave been assigned to frequency groups. As such, frequency groups neednot contain equal numbers of channels. From FIG. 3, the cause ofadjacent channel interference (between sectors of cellular communicationsystems) is represented by boundaries (e.g. 98 and 99) betweencontiguous frequency channels.

A typical channel assignment protocol is illustrated in TABLE 1immediately below:

TABLE 1 Frequency Group a1 b1 c1 d1 e1 f1 Channel Number 1, 13 2, 143,15 4, 16 5, 17 6, 18 Frequency Group a2 b2 c2 d2 e2 f2 Channel Number7, 19 8, 20 9, 21 10, 22 11, 23 12, 24

It will be noted that TABLE 1 assumes that the frequency bandwidth 90 issufficient to support 24 channel (carriers), such as Broadcast ControlChannels (BCCH).

Now, turning to FIG. 4, a preferred embodiment of the present inventionproduces a two-site repeat (re-use) pattern that optimally balances theco-channel and adjacent channel interference levels. As can be seen, acoverage area 120 is defined by a number of conventionally-representedhexagonal sites (or cells) each having six sectors. Referring toadjacent sites 130 and 132 (which have been outlined in bold tofacilitate recognition and which together form a cluster 134 that isrepeated to produce a mosaic for the coverage area 120), the totalcombined number of sectors (namely, twelve sectors) in these two siteseach receive a unique frequency group. As such, each cluster requirestwelve distinct frequency groups, with the communication systemtherefore requiring at least twelve distinct (contiguous) channels.Furthermore, in the preferred embodiment, channels are assigned tofrequency groups in accordance with the assignment protocol tabulatedabove. However, it will be appreciated that there need only be a numberof frequency channels corresponding to the number of sectors in thecluster 134, i.e. a minimum of twelve frequency channels are requiredfor a cluster having two six-sectored sites. For the sake ofillustration, another cluster 136 has been identified and specificallyoutlined in the coverage area 120 of FIG. 4.

In the preferred embodiment, each site (e.g. site 130) contains a basestation and associated infrastructure (not illustrated for the sake ofclarity) that is similar to that described in relation to FIGS. 1 and 2,and as will be understood. However, unlike the side-illumination sectorcoverage provided by each individual RCU of FIG. 2, each RCU for eachsector in FIG. 4 is arranged to provide a corner-illumination of itsrespective sector, whereby each side of the hexagonally represented siteis partitioned between frequency groups.

With regard to the arrangement of frequency groups (a1-f1 and a2-f2)containing consecutive (and perhaps contiguous) frequency channels ofincrementing frequency (as illustrated in FIG. 3 and tabulated inTABLE 1) in each cluster, a first frequency group a1 (having the lowestfrequency channel) and a last frequency group f2 (having the highestfrequency channel) are nominally disregarded with respect to placementwithin particular sectors of the cluster (e.g. cluster 134). Theremaining frequency groups b1-f1 and a2-e2 are then nominally pairedtogether by associating adjacent frequency groups, i.e. b1 is associatedwith c1; d1 is associated with e1; f1 is associated with a2; b2 isassociated with c2; and d2 is associated with e2.

A first member (the lowest frequency member) of each of these pair offrequency groups is uniquely assigned to a particular sector of a firstsite (e.g. site 130) of the cluster 134, whereas a second member (thehigher frequency member) of each of these pairs of frequency groups isassigned to a corresponding (identically located/positioned) sector of asecond site (e.g. site 132) of the cluster 134. Assignment of thefrequency group pairings continues on an adjacent and rotational basissuch that all first members are assigned to the first site 130 and allsecond members are assigned to the second site 132, and each first orsecond member of the pair is side adjacent to at least one other nextbut one adjacent frequency group, i.e. on a sectorial basis b1 is sideadjacent to d1 which is side adjacent to f1 which is side adjacent to b2which is side adjacent to d2 (in a first site 130), and c1 is sideadjacent to e1 which is side adjacent to a2 which is side adjacent to c2which is side adjacent to e2 (in a second site 132).

By following this frequency group placement pattern, an empty sectorthat is side adjacent to both b1 and d2 in site 130 appears and,similarly, an empty sector that is side adjacent to both c1 and e2 insite 132 also appears. Therefore, ten of the twelve possible sectors incluster 134 are filled by the pairings, with the remaining two sectorsreceiving a pairing of the first frequency group a1 (having the lowestfrequency channel) and the last frequency group f2 (having the highestfrequency channel). More particularly, the first frequency group a1 isinserted into the empty sector of site 132 such that it is only sideadjacent to c1, i.e. a next but one adjacent frequency group. Then, bydefault, the last frequency group f2 is inserted into the empty sectorof site 130 such that it is only side adjacent to d2, i.e. a next butone adjacent frequency group. In this way, an interface 136 betweensites of the cluster 134 (or 136) of the preferred embodiment willcontain: (a) side adjacencies between (i) frequency group f2 andfrequency group d2 (in site 130) and (ii) frequency group e1 andfrequency group a2 (in site 132); and (b) half-side adjacencies between(i) frequency group f2 and frequency group e1 (in different sites) and(ii) frequency group d2 and frequency group e2 (also in differentsites). As such, each frequency group pairing (containing adjacentchannels that could potentially cause splatter if placed side adjacentto each other) is separated by a distance (diameter) of at least onesite.

In the new two-site repeat pattern of FIG. 4, no adjacent frequencygroups are found within the clusters (134, 136), and adjacent channelinterference (splatter) is substantially reduced. However, since thephysical separation of co-channel sectors is reduced in comparison with,for example, a four-site use pattern, a carrier-to-interference ratio(C/I) measurement for the co-channel of the preferred embodiment isreduced but nonetheless still provides adequate isolation.

Consequently, the preferred embodiment of the present inventionadvantageously provides a low-cost, two-site repeat pattern offering lowadjacent channel interference and adequate co-channel isolation, whileproviding the inherent efficiency advantages associated with a reducedrepeat pattern.

In an alternate (but less efficient) two-site repeat pattern (shown inFIG. 5), a coverage area 150 again contains a plurality of sites(represented in conventional hexagonal format) each having a basestation and associated infrastructure (as will be understood). However,unlike the preferred embodiment in which sectors are corner-illuminatedby individual RCUs, each RCU for each sector in FIG. 5 is arranged toprovide a side-illumination of its respective sector.

With respect to the arrangement of frequency groups (a1-f1 and a2-f2) ineach cluster of FIG. 5, a first frequency group a1 (having the lowestfrequency channel) and a last frequency group f2 (having the highestfrequency channel) are initially disregarded with respect to placementwithin particular sectors of a cluster. The remaining frequency groupsb1-f1 and a2-e2 are then again nominally paired together by associatingadjacent frequency groups, i.e. b1 is associated with c1; d1 isassociated with e1; f1 is associated with a2; b2 is associated with c2;and d2 is associated with e2.

The assignment of the frequency group pairings to corresponding sectorsin each two-size cluster again occurs in an fashion identical to thatpreviously described for FIG. 4, namely that members of each frequencygroup pairing are split between sites in the cluster and each first orsecond member of the pair is then positioned in a sector that is sideadjacent to at least one other next but one adjacent frequency group.However, in the side-illuminated configuration of the alternateembodiment, interference points 152-160 (at each corner of every site)appear between adjacent frequency groups, e.g. b1 and a1 or d1 and c1 ord2 and c2, resulting in a lower isolation for adjacent channel splatter(which may be overcome by adhering to strict hand-over regimes betweensites, as will be appreciated by the skilled addressee). In thisrespect, it is noted that the interference experienced at interferencepoints 152-160 in FIG. 5 arises between only two frequency groups (orchannels) that potentially interfere at each point. Therefore, hand-offto a base station responsible for any one of the four othernon-interfering frequency groups (or channels) that also converge atthat interference point could be acceptable.

In summary, a cellular communication system (configured as describedimmediately above) has a frequency bandwidth arranged into a pluralityof frequency channels that are allocated on a consecutive frequencybasis to corresponding sectors in each of neighbouring first and secondsites. The present invention may be employed with any cellularcommunication system, such as time-division multiplexed systems(including those capable of supporting frequency hopping, if desired).

With respect to a cellular communication system configured to support anunderlay/overlay frequency re-use, a typical channel assignment protocolis illustrated in TABLE 2 immediately below:

TABLE 2 Frequency Group b1 b2 b3 b4 b5 b6 Channel Number 1,19 2,20 3,214, 22 5, 23 6, 24 Frequency Group b7 b8 b9 b10 b11 b12 Channel Number 78 9 10 11 12 Frequency Group t1 t2 t3 t4 t5 t6 Channel Number 13 14 1516 17 18

It will be noted that TABLE 2 assumes that the frequency bandwidth 90(of FIG. 3) is sufficient to support 24 channels (carriers).

In the context of a cellular communication system employing concentriccells, FIG. 6 shows a typical allocation of frequency bandwidth. In thisparticular instance, a first group (typically containing a plurality offrequency channels) 200 is assigned for use in a first repeat pattern,whereas a second group (containing a different plurality of frequencychannels) 202 is assigned for use in a second repeat pattern. Forexample, the group 200 may be BCCHs, whereas the second group 202 may beTCHs. Moreover, although an actual number of channels in each of thegroups (200 and 202) may be the same, it is likely that a greater numberof channels will be assigned to the more frequently (smaller) repeatedpattern in order to avoid interference as far as possible, as will beunderstood.

Indeed, frequency groups used for BCCHs will probably contain a singlefrequency channel, while frequency groups used for TCHs will probablycontain multiple frequency channels.

It will of course be appreciated that FIG. 6 represents one particularallocation scheme, and that the allocation of frequency channels needonly satisfy cell planning requirements (with respect to carrierinterference) for frequency groups within the cellular system. Forexample, frequency channels may be assigned to the different repeatpatterns on a rotational basis, whereby a specified number ofconsecutive (potentially contiguous) frequency channels are allocated tothe first group before the remaining frequency channels are allocated(on another rotational basis) to the second group, as illustrated inTABLE 3 below:

TABLE 3 Frequency Group t1 t2 t3 t4 t5 t6 Channel Number 1, 7 2, 8 3, 94, 10 5, 11 6, 12 Frequency Group b1 b2 b3 b4 b5 b6 Channel Number 13 1415 16 17 18 Frequency Group b7 b8 b9 b10 b11 b12 Channel Number 19 20 2122 23 24

Turning to FIG. 7, another embodiment of the present invention producesa two-site repeat (re-use) pattern with a one-site underlay/overlayrepeat scheme that optimally balances the co-channel and adjacentchannel interference levels. As can be seen, a coverage area 220 isdefined by a number of conventionally-represented hexagonal sites (orcells) each having six sectors. As will be appreciated, this hexagonaltopography has the advantage of keeping interference relative low. Now,referring to adjacent sites 230 and 232 (which have been outlined inbold to facilitate recognition and which together form a cluster 234that is repeated to produce a mosaic for the coverage area 220), it canbe seen that eighteen distinct frequency groups have been distributedbetween the twelve sectors. More specifically, the eighteen frequencygroups (each of which may contain one or more frequency channels) areattributed to either a two-site (by six-sector) repeat pattern or aone-site (by six-sector) repeat pattern, with the two-site repeatpattern receiving twelve of the frequency groups (b1-b2) and theone-site repeat pattern receiving the remaining six frequency groups(t1-t6). Furthermore, in this embodiment, frequency groups b1-b12 arenominally BCCHs, while frequency groups t1-t6 are nominally TCHs. Forthe sake of illustration, another cluster 236 has been identified andspecifically outlined in the coverage area 220 of FIG. 7.

Each site (e.g. site 230) contains a base station and associatedinfrastructure (not illustrated for the sake of clarity) that is similarto that described in relation to FIGS. 1 and 2, and as will beunderstood. However, unlike the side-illumination sector coverageprovided by each individual RCU of FIG. 2, each RCU for each sector inFIG. 7 is arranged to provide a corner-illumination of its respectivesector, whereby each side of the hexagonally represented site ispartitioned between frequency groups.

With regard to the arrangement of frequency groups b1-b2 (containingconsecutive and potentially contiguous frequency channels ofincrementing frequency) in the two-site repeat pattern of cluster 234, afirst frequency group b1 (having the lowest frequency channel) and alast frequency group b12 (having the highest frequency channel) arenominally disregarded with respect to placement within particularsectors of the cluster (e.g. cluster 234). The remaining frequencygroups b2-b11 are then nominally paired together by associating adjacentfrequency groups, i.e. b2 is associated with b3; b4 is associated withb5; b6 is associated with b7; b8 is associated with b9; and b10 isassociated with b12.

A first member (the lowest frequency member) of each of these pairs offrequency groups is uniquely assigned to a particular sector of a firstsite (e.g. site 230) of the cluster 234, whereas a second member (thehigher frequency member) of each of these pairs of frequency groups isassigned to a corresponding (identically located/positioned) sector of asecond site (e.g. site 232) of the cluster 234. Assignment of thefrequency group pairings continues on an adjacent and rotational basissuch that all first members are assigned to the first site 230 and allsecond members are assigned to the second site 232, and each first orsecond member of the pair is side adjacent to at least one other nextbut one adjacent frequency group, i.e. on a sectorial basis b3 is sideadjacent to b5 which is side adjacent to b7 which is side adjacent to b9which is side adjacent to b11 (in a first site 230), and b2 is sideadjacent to b4 which is side adjacent to b6 which is side adjacent to b8which is side adjacent to b10 (in a second site 232).

By following this frequency group placement pattern, an empty sectorthat is side adjacent to both b3 and b11 in site 230 appears and,similarly, an empty sector that is side adjacent to both b2 and b10 insite 232 also appears. Therefore, ten of the twelve possible sectors incluster 234 are filled by the pairings, with the remaining two sectorsreceiving a pairing of the first frequency group b1 (having the lowestfrequency channel) and the last frequency group b12 (having the highestfrequency channel). More particularly, the first frequency group b1 isinserted into the empty sector of site 230 such that it is only sideadjacent to b3, i.e. a next but one adjacent frequency group. Then, bydefault, the last frequency group b12 is inserted into the empty sectorof site 232 such that it is only side adjacent to b10, i.e. a next butone adjacent frequency group. In this way, an interface 237 betweensites of the cluster 234 (or 236) of this particular configuration willcontain: (a) side adjacencies between (i) frequency group b1 andfrequency group b11 (in site 230) and (ii) frequency group b4 andfrequency group b6 (in site 232); and (b) half-side adjacencies between(i) frequency group b1 and frequency group b5 (in different sites) and(ii) frequency group b11 and frequency group b6 (also in differentsites). As such, each frequency group pairing (containing adjacentchannels that could potentially cause splatter if placed side adjacentto each other) is separated by a distance (diameter) of at least onesite.

Having regard to the one-site underlay/overlay of the remaining sixfrequency groups t1-t6, each sector of each site contains one of thesefrequency groups (and hence one or some traffic frequency channels),with corresponding sectors in adjacent site containing the sameparticular frequency group. The frequency groups t1-t6 are cyclicallyarranged in the sectors of sites 230 and 232, for example, on thefollowing adjacent basis: t1 is adjacent to t3 which is adjacent to t5which is adjacent to t2 which is adjacent to t6 which is adjacent to t4which is adjacent to t1. Therefore, a next but one frequency group (andhence frequency channel) relationship with at least one adjacent sectorexists between half of the adjacent frequency (TCH) groups in a site.Furthermore, the interleaving of frequency groups t2, t6 and t4 (in anext but three frequency channel relationship) is necessitated by theneed to avoid adjacent channel interference in the one-site repeatpattern.

In overview of each cluster of FIG. 7, no adjacent frequency groups arefound within the two-site repeat pattern, and adjacent channelinterference (splatter) in the two-site repeat pattern is substantiallyreduced. However, since the physical separation of co-channel sectors isreduced in comparison with, for example, a four-site use pattern, acarrier-to-interference ratio (C/I) measurement for the co-channel ofthis particular embodiment is reduced but nonetheless still providesadequate isolation. With regard to the one-site underlay, no adjacentboundaries between frequency groups t1-t6 can be found within a site,while co-channel interference between frequency groups t1-t6 at theperiphery of each sector within a site can be ignored because the thisembodiment of the present invention contemplates the implementation of ahand-over (orchestrated principally by the base station in response tomobile unit or base-station determined measurements known in the art)from the frequency groups of the one-site (TCH) repeat pattern to thefrequency groups two-site (BCCH) repeat pattern. More specifically, oncea call on a TCH carrier suffers from unacceptably high interference, thecall will be handed over to the channels available on the co-sited BCCH.As such, this particular embodiment of the present invention utilises anoptimal intra-cell hand-over algorithm to reduce overall “outrage” andto therefore maintain a relatively high quality of service. Indeed, itcan be seen that the “outage” (i.e. a predefined threshold for thesignal to interference ratio) for the one-site repeat pattern is greaterthat the outage of the BCCH two-site repeat pattern because the distancebetween co-channels in the one-site repeat pattern is only one site(cell) diameter.

Therefore, this underlay/overlay configured embodiment of the presentinvention advantageously provides a low-cost, two-site repeat patternwith one-site overlay/underlay offering low adjacent channelinterference (because no adjacent channels on the same site share acommon boundary) and adequate co-channel isolation, while providing theinherent efficiency advantages associated with a reduced repeat pattern.Moreover, the two-layer scheme that arises from the differingtopographies of the underlaid/overlaid configuration provides anincreased trunking efficiency in that more carriers are allocated toeach site, and an improved (higher) in-building signal penetration thatarises because of an ability to implement a narrower antenna beamwidthfor the hexagonal cell-structure of the present invention (rather than arelatively wide antenna beamwidth for a three-sector site), as will beunderstood by the skilled addressee.

In an alternate (but less efficient) two-site repeat pattern having aone-site underlay/overlay scheme (shown in FIG. 8), a coverage area 250again contains a plurality of sites (represented in conventionalhexagonal format) each having a base station and associatedinfrastructure (as will be understood). However, unlike thecorner-illuminated sectors of the previously describedunderlaid/overlaid configuration, each RCU for each sector in FIG. 8 isarranged to provide a side-illumination of its respective sector.

With respect to the arrangement of frequency groups (b1-b12 and t1-t6)in each cluster of FIG. 8, a first frequency group b1 (having the lowestfrequency channel) and a last frequency group b12 (having the highestfrequency channel) are initially disregarded with respect to placementwithin particular sectors of a cluster. The remaining frequency groupsb2-b11 are then again nominally paired together by associating adjacentfrequency groups, i.e. b2 is associated with b3; b4 is associated withb5; b6 is associated with b7; b8 is associated with b9; and b10 isassociated with b11.

The assignment of the frequency group pairings to corresponding sectorsin each two-site cluster again occurs in an fashion identical to thatpreviously described for FIG. 7, namely that members of each frequencygroup pairing are split between sites in the cluster and each first orsecond member of the pair is then positioned in a sector that is sideadjacent to at least one other next but one adjacent frequency group.However, in the side-illuminated configuration of this alternateunderlaid/overlaid embodiment, interference points (at each corner ofevery site) appear between adjacent frequency groups, e.g. b4 and b3 orb10 and b9 or b6 and b5, resulting in a lower isolation for adjacentchannel splatter (which may be overcome by adhering to strict hand-overregimes between sites, as will be appreciated by the skilled addressee).In this respect, it is noted that the interference experienced atinterference points in FIG. 8 arises between only two frequency groups(or channels) that potentially interfere at each point. Therefore,hand-off to a base station responsible for any one of the four othernon-interfering frequency groups (or channels) that also converge atthat interference point could be acceptable.

Frequency groups t1-t5 (in the one-site overlay/underlay repeat pattern)are assigned in an identical way to that previously described inrelation to FIG. 5, so recitation has been ignored for the sake ofbrevity.

It will, of course, be understood that the above description has beengiven by way of example only and that modifications in detail, such asthe orientation of each cluster and the rotational assignment of eachfrequency group pairing to particular but corresponding sectors in eachcluster, may be made within the scope of the present invention.Furthermore, with respect to an available frequency bandwidth for thecommunication system, this may be constructed from two or more separateblocks of spectrum, in which blocks have some channels that havefrequencies contiguous to one another. Additionally, since the one-siteoverlay/underlay offers increased capacity, not all sites in the systemneed contain this overlay/underlay scheme. For example, sites in ruralareas may adequately cope with communication traffic using the two-siterepeat pattern only. As such, it is envisaged that the one-site(overlay/underlay) repeat pattern may be selectively applied to areas ofspecific need within the cellular communication system. Furthermore, thepresent invention may be employed with any cellular communicationsystem, such as time-division multiplexed systems (including thosecapable of supporting frequency hopping, if desired).

What is claimed is:
 1. A cellular communication system having afrequency bandwidth arranged into a plurality of frequency channels, thecellular communication system comprising neighbouring first and secondsites each having sectors containing at least one frequency channelcomprising a frequency group, wherein corresponding sectors in each ofthe neighbouring first and second sites have consecutive frequencychannels from the frequency bandwidth forming the frequency groups, thefrequency bandwidth arranged into the plurality of frequency channelsfor use in a two-site repeat pattern (b1-b12) and a one-site repeatpattern (t1-t6) of the cellular system further comprising thoseneighbouring first and second six-sectored sites in which each sectorcontains at least one frequency channel from the two-site repeat patternand at least one frequency channel from the one-site repeat pattern,wherein the corresponding sectors in each of the neighboring first andsecond six-sectored sites have consecutive frequency channels of thetwo-site repeat pattern and identical frequency channels of the one-siterepeat pattern.
 2. The cellular communication system according to claim1, wherein neighbouring sectors within both the first and second sitesobserve a next but one frequency channel relationship with adjacentfrequency channels in at least one neighbouring sector.
 3. The cellularcommunication system according to claim 1 or 2, wherein the first andsecond sites are corner-illuminated sites.
 4. The cellular communicationsystem according to claim 1 or 2, wherein the first and second sites areside-illuminated sites.
 5. The cellular communication system accordingto claim 1, wherein neighbouring sectors within both the first andsecond six-sectored sites observe a next but one frequency channelrelationship with adjacent frequency channels in at least oneneighbouring sector for the two-site repeat pattern.
 6. The cellularcommunication system according to claim 1, wherein a first half to heneighbouring sectors within both the first and second six-sectored sitesobserve a next but one frequency channel relationship with adjacentfrequency channels in at least one neighbouring sector for the one-siterepeat pattern and a second half of the neighbouring sectors within boththe first and second six-sectored sites observe a next but threefrequency channel relationship with adjacent frequency channels in atleast one neighbouring sector for the one-site repeat pattern.
 7. Thecellular communication system according to claim 1, wherein the two-siterepeat pattern overlays the one-site repeat pattern.
 8. The cellularcommunication system according to claim 6, further comprising means forcontrolling hand-over of a call from the one-site repeat pattern to thetwo-site pattern in response to a relatively high interference level forthe call on the one-site repeat pattern.
 9. The cellular communicationsystem according to claim 1, wherein the plurality of frequency carriersare allocated to the one-site repeat pattern and the two-site repeatpattern on a cyclic basis.
 10. The cellular communication systemaccording to claim 1, wherein the frequency channels of the two-siterepeat pattern are BCCHs and the frequency channels of the one-siterepeat pattern are TCHs.
 11. A cellular communication system having afrequency bandwidth arranged into a plurality of frequency channels thatare distributed on a consecutive frequency channel basis amongst aplurality of frequency groups the cellular communication systemcomprising a cluster having neighbouring first and second six-sectoredsites, each sector of each site containing a frequency group having atleast one frequency channel and wherein: the first six-sectored sitecomprises a first frequency channel and at least five other frequencychannels each having an integer multiple next but one frequency channelrelationship to the first frequency channel, wherein neighbouringsectors in the first six-sectored site each contain a frequency grouphaving respective frequency channels that observe a next but onefrequency channel relationship with frequency channels in at least oneneighbouring frequency group; the second six-sectored site comprises asecond frequency channel, consecutive in frequency to the firstfrequency channel, and at least five other frequency channels eachhaving an integer multiple next but one frequency channel relationshipto the second frequency channel, wherein neighbouring sectors in thesecond six-sectored site each contain a frequency group havingrespective frequency channels that observe a next but one frequencychannel relationship with at least one neighbouring frequency group; andconsecutive frequency channels of the frequency bandwidth are assignedto corresponding sectors in each of the neighbouring first and secondsix-sectored sites to produce a two-site re-use pattern for the clusterin which consecutive frequency channels are alternated between the firstand second six-sectored sites.
 12. The cellular communication systemaccording to claim 11, wherein at least one frequency group containsmore than one carrier frequency obeying the integer multiple next butone frequency channel relationship.
 13. The cellular communicationsystem according to claim 12, wherein the plurality of frequencycarriers are allocated to the frequency groups on a cyclic basis. 14.The cellular communication system according to claim 11, 12 or 13,wherein the first and second six-sectored sites are corner-illuminatedsites.